Light source apparatuses using a short-arc high pressure discharge lamp in combination with a reflector are employed as backlights of projectors, projection TVs, and so forth.
In recent years, there has been a demand for these high pressure discharge lamps with respect to the improvement in properties such as further enhancement in brightness, reduction in size, and longer lifetime. Particularly, the longer lifetime is highly desired, further improvement of which is required. In this regard, in order to extend the lifetime, it is an important issue to maintain the arc length during the lifetime. More specifically, the driving voltage (hereinafter, referred to as a “lamp voltage”) of the high pressure discharge lamp needs to be maintained at a constant level.
For this reason, these high pressure discharge lamps are filled with mercury and a minute amount of halogen. By the halogen cycle, tungsten that is a material for an electrode evaporated during driving returns to a tip of the electrode. This suppresses the fluctuation in arc length during the lifetime, thereby maintaining the lamp voltage.
In fact, however, it is known that the lamp voltage decreases at the initial period of approximately several tens of hours of accumulative driving time of the high pressure discharge lamp, while the lamp voltage increases for a while during the subsequent long lifetime.
Additionally, the lamp voltage also shows behaviors such as increase and decrease during the lifetime due to the variation among individual lamps and the variation in driving condition such as the outside temperature.
However, it is difficult to control these fluctuations in lamp voltage under the same driving frequency condition. For this reason, a proposal is made to achieve the improvements by changing the frequency. One example is a method for controlling the lamp voltage by changing the driving frequency in accordance with the lamp voltage while the lamp is driven, as described in Patent Document 1. Specifically, the driving frequency is controlled to be increased when the lamp voltage falls below a certain reference value, while the driving frequency is decreased when the lamp voltage exceeds a certain reference value. This is the control based on the known fact that the lamp voltage tends to increase in its behavior when the lamp-driving frequency is high, whereas the lamp voltage tends to decrease in its behavior when the driving frequency is low (hereinafter, respectively referred to as a “high frequency” and a “low frequency”).
Further, as another countermeasure, proposed is a control in which the driving frequency is changed by switching among two or more different values multiple times to drive a lamp, as in Patent Document 2, for example. Specifically, a lamp current waveform is employed which is synthesized from multiple frequency components including the high frequency component and the low frequency component in a predetermined balance from the beginning. Thus, the advantage of the high frequency and the advantage of the low frequency are to be demonstrated together.
More specifically, a square wave alternating current that is a combination of multiple driving frequencies shown in FIG. 7 is applied to drive a high pressure discharge lamp. Further, FIG. 8A is a graph showing the relationship between accumulative driving time and a luminance maintenance rate in the driving test. FIG. 8B is a graph showing the relationship between the accumulative driving time and the lamp voltage in the driving test. According to the result of this test, the high pressure discharge lamp is designed to be driven while multiple driving frequencies are selected appropriately and the behavior of the lamp voltage and the combinations of the driving frequencies are switched so as to achieve the preferable the luminance maintenance rate during the lifetime of the lamp and behavior of the lamp voltage.    Patent Document 1: Japanese Patent Application Publication No. 2006-185663    Patent Document 2: Japanese Patent No. 3851343